How do you find the offset of a member within a struct?

Use offsetof(struct T, member) from <stddef.h>, which gives the byte distance from the struct's start to that field (accounting for padding).

#include <stddef.h>

struct example {
    // offset 0
    char a;
    // offset 4 (after 3 bytes padding)
    int b;
    // offset 8
    char c;
};

// 0
size_t off_a = offsetof(struct example, a);
// 4
size_t off_b = offsetof(struct example, b);
// 8
size_t off_c = offsetof(struct example, c);

Use case: Convert member pointer back to struct pointer

#define container_of(ptr, type, member) \
    ((type *)((char *)(ptr) - offsetof(type, member)))

struct node {
    int data;
    struct node *next;
};

void process(int *data_ptr) {
    // Get the struct that contains this data member
    struct node *n = container_of(data_ptr, struct node, data);
    printf("Next: %p\n", n->next);
}

This is how Linux kernel linked lists work!

Manual offsetof (how it works internally):

#define my_offsetof(type, member) \
    ((size_t)&((type *)0)->member)
// Takes address of member in a struct at address 0